I was a Quora Top Writer in 2012 and 2013, and I've also built a lot of web stuff and spent a lot of time studying web communities. Purely from a craftsmanship perspective, they've done an incredible job. The visual design, the interaction design, the writing and reading experiences, the social incentives, the notification system: all of it is top notch.
That said, I have never understood what their economic plan is. I have a very good understanding of the economics of content businesses, and the people at Quora are either incredible geniuses or totally nuts. Since they won't say how they plan to make money, we can't say. If I had to bet, It be "totally nuts", just because so many content businesses get seduced by a creative vision that current revenue streams just can't support. But having met some of them, I know they're very smart, so I am hoping that I am wrong and that they have some clever monetization strategy that will let them justify their very large valuation.